Seven days of warm water immersion modulate baseline fluid-regulatory hormones without altering acute multisystem physiological stress responses in older males

Older adults exhibit heightened vulnerability to heat-related morbidity, yet it remains unclear whether short-term heat acclimation modifies integrative physiological stress responses in aging. We tested whether 7 days of passive heat acclimation via warm water immersion alters basal concentrations and acute exercise-heat stress responses of biomarkers reflecting fluid regulation, gastrointestinal epithelial integrity, renal stress, and inflammation in older males. Twelve habitually active older males (median [interquartile range] age: 68 [64-73] yr; [Formula: see text]: 34.1 [29.4-36.1] mL O₂·kg⁻¹·min⁻¹) completed an exercise-heat stress test before and after 7 consecutive days of warm water immersion (∼40°C), with rectal temperature maintained at ∼38.5°C for the final 60 min. Blood samples were collected before and after each exercise-heat stress test and analyzed using linear mixed-effects models (day × time). Repeated warm water immersion increased resting renin by 68% [95% confidence interval: 29, 120; P < 0.001], whereas neutrophil gelatinase-associated lipocalin and lipopolysaccharide-binding protein decreased by 24% [-40, -4; P = 0.023] and 22% [-37, -3; P = 0.026], respectively. Exercise-heat stress increased aldosterone by 45% [12, 87%; P = 0.004], renin by 41% [8, 84%; P = 0.01], intestinal fatty acid-binding protein by 52% [26, 85%; P < 0.001], and plasma osmolality by 5.6% [2.5, 8.7%; P < 0.001]. However, there was no day × time interaction for any biomarker (all P > 0.05), indicating that 7 days of warm water immersion did not alter the magnitude of the exercise-heat stress response. Short-term passive heat acclimation in older males induces selective fluid-regulatory endocrine adaptations consistent with preserved renin-angiotensin-aldosterone system plasticity, whereas gastrointestinal and renal stress responses to exercise-heat exposure remain unchanged.NEW & NOTEWORTHY This study demonstrates that short-term (7 days) passive heat acclimation via warm water immersion induces selective endocrine adaptation without uniform protection across physiological systems in older males aged 60-80. Although RAAS-mediated fluid regulation was modified following heat acclimation, gastrointestinal and renal stress responses to acute heat exposure persisted. These findings suggest that physiological adaptation to repeated heat exposure in aging may be compartmentalized across organ systems, with implications for vulnerability during extreme heat events.